In the manufacture of vehicle body parts or other large-surface, spatially formed sheet metal parts, the first press step, in most cases, uses a drawing press that imparts a so far plane blank with a three-dimensional form. This is accomplished in a drawing tool that holds the rim of the blank by clamping it in place, or that also allows it to slide in a controlled manner toward the center of the metal sheet, while the part of the metal sheet circumscribed by the sheet metal blank holder receives the desired three-dimensional form between a matrix and a punch.
Today, established drawing presses comprise a punch that is statically supported by a press table, and the associate matrix is held on the ram that can be moved vertically up and down. During the drawing operation, blank holder encloses the ram and is pressed downward by the rim of the matrix against the force of a drawing cushion. Referring to this basic configuration, the convexly curved sheet metal side is formed on the upper side of the sheet metal part, as is also desired for the subsequent press steps. During the subsequent press steps, in particular punching operations are also performed. In the case of vehicle body parts, it is necessary, as a rule, that the resultant punch burr be located on the hollow side, i.e., the concavely curved underside of the sheet metal part. Inasmuch as reversing stations and the like between the individual press steps must be considered unacceptable, the design form addressed here has established itself as the standard. Consequently, design forms with the matrix located on the bottom and the punch located on the top (as well as with the sheet metal holder located on the top) as have been known, for example, from publication DE 10117578 B4, are thus less frequently used.
Presses of the aforementioned type with the matrix at the top and the statically supported punch on the bottom have been known, for example, from publication DE 10 2006 025271 B3. In this press, the ram, as well as the drawing cushion, are driven by servomotors via spindle-type lifting gear. After each, the ram and the drawing cushion, performs a back and forth movement, the servomotors must perform a reversal of movement. The reversal of movement occurs at the respective dead center of the movement of the drawing cushion or of the ram. This means that the deceleration and acceleration phases of the servomotors noticeably extend the cycle time required for drawing a sheet metal part.
In addition, there is a considerable use of energy in such presses. Considerable force is required for depressing the blank holder, namely for overcoming the blank holding force. This force cannot be randomly reduced, on the contrary, it must increase with increasing sheet metal strength. Considering this design, the path to be traveled by the blank holder cannot either be randomly reduced because it essentially corresponds to the drawing depth and thus is prespecified by the geometric configuration of the workpiece. Even if the energy converted by the drawing cushion can optionally be re-supplied to a storage, a net or another user, energy losses are nearly inevitable.